The invention relates to a vibration switch, particularly but not exclusively for use in security systems to detect vibration through building structures during a forcible entry.
The operation of such switches relies on an electrical connection being maintained by means of gravity acting upon a free moving mass connected to or part of the electrical circuit. The body of the switch is normally firmly attached to the building structure. During vibration, the switch body and associated fixed electrical contacts will move, whereas the mass will tend to remain relatively stationary due to inertia effects.
During vibration, the electrical circuit will be opened and closed rapidly as the mass loses contact with its points of rest. The electrical signals obtained may be suitably analysed and processed by electronic circuits and, provided pre-set conditions are met, be used to signal an alarm condition.
Numerous different switch designs are in use but most suffer from certain disadvantages. It is known to provide a sensor switch with conductive circular cross-section bars as bridging conductors between a pair of contacts which are perpendicular to the bars, the resulting electrically parallel connections between the contacts being disturbed when the sensor is vibrated due to relative movement of a mass supported by the bars. U.S. Pat. No. 4,686,335 discloses an arrangement in which the force applied to the bars is increased by a leverage principle, allowing use of a smaller mass. However, experience has shown that occasional high resistance or open-circuit conditions at the contact points may be caused by microscopic particles of foreign matter, oxides or polymer contamination from impurities migrating through the relatively porous gold contact plating which is often used.
The present invention seeks to overcome the problem of occasional false alarm conditions due to contact contamination.